# Questions tagged [general-relativity]

Questions about relativistic theory of gravity. General relativity describes gravity as a geometrical property of space and time.

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### Is gravitational lensing "additive" in a line of sight?

As usually depicted, gravitational lensing implies, at least, three objects: a distant source, an observer, and a "massive enough" object in the middle, where enough is the mass needed to ...
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### What does it exactly mean for spacetime to have no global symmetries?

Are there spacetimes or metrics with no global symmetries? Spacetimes/metrics with no global Poincaré, Lorentz, diffeomorphism, CPT, translational and gauge invariances? And if there are, what does it ...
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### Why does the outside edge of a Kerr black hole ergosphere rotate at the same speed as the inner edge?

Reading through the Wikipedia on a Kerr black hole ergosphere, and this nice question on the maximal spin rate of a black hole leads me to ask: why does the outer edge of the ergosphere rotate at the ...
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### On the geodesics of the metric $ds^2=-\rho^2d\alpha^2+d\rho^2$ and the constant $l=\rho^2\frac{d\alpha}{d\tau}$

In my assignment I have to deal with the 2D spacetime metric $$ds^2=-\rho^2d\alpha^2+d\rho^2.$$ During this assignment we have shown that the constant $$l=\rho^2\frac{d\alpha}{d\tau}$$ is a constant ...
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### About the strange case of Bonnor beams

Bonnor beams are light beams of infinite extent and small thickness. They have an associated spacetime (related to their energy and momentum which shapes the stress-energy tensor) of which Bonnor ...
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### Gravitational recoil with stars/planets...?

When two black holes are merging, the resulting merge can be ejected if one of the black holes had less mass than the other one, so the gravitational waves emitted by both of them is unbalanced, and ...
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### Kinetic energy in cosmology?

Spacetime expands at an accelerated rate due to the Hubble flow. In many papers that I've read, objects coupled to the Hubble flow are treated as if they have some velocity and kinetic energy ...
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### Deviations of conservation laws in the context of cosmological evolution?

If energy is "not conserved" in General Relativity (or at least, it is difficult to define it) in the context of an accelerating expanding spacetime (like it happens in our Universe), are ...
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### Does the presence of dark matter in the Milky Way significantly distort our parallax based measurements of distance?

According to accepted astronomical wisdom, dark matter represents a large fraction of the total mass of the Milky Way. And thanks to Papa Einstein, we know that light rays can be bent when passing ...
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### Does General Relativity predict anything about the creating of a singularity, once enough matter collapses inward to form an event horizon?

Ignoring quantum gravity, which we don't know much about, current theory (GR) states that an event horizon forms when there is enough matter/energy density to create it, and logically, a singularity I ...
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### Is TDB monotonic? Does it or any of its derivatives have yearly discontinuities with respect to an atomic clock?

This in-depth answer mentions that ...TDB is updated every year. As technology improves, these subtle updates change the timing of past events. This makes me wonder... Does this updating result ...
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### Which static spherically symmetric perfect fluid solutions of Einstein field equations are known to be suitable for stellar models?

Wikipedia's (Static spherically symmetric perfect fluid) short list ends in 2005 and lists only in total 5 such solutions. Does somebody know more?
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### Parker Solar Probe and clock correction

Due to the dynamic changes in speed and the value of the gravitational field in which the Parker Solar Probe stays, the clocks should have the errors due to relativistic effects. https://physics....
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### Order of magnitude of this phase space invariant?

So I wanted know the order of magnitude of this phase space invariant (equation $3.31a$ )$R$ in our universe (FLRW metric): $$R = \int_{p} \frac{\mathcal{N}(x,p,t)}{\mathcal{E}} dp_x dp_y dp_z$$ ...
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### How String Theory calculates the Entropy of the Black Hole in a Background dependent way?

How does string theory get an agreement to the Hawking-Bekenstein equations to calculate the quantum entropy of a black hole in a background dependent way? Is there any sort of area parameter in ...
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### If time is not absolute and depends on the gravitational field, then why we keep using our own (particular) time in cosmology?

Sorry if my question is not really clear, but indeed it is not clear in my mind neither. But maybe some of you can help. Time is not absolute and can be affected by the gravitational potential. Now ...
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### How to compare the observation with the theoretically predicted result?

On the Wikipedia Article on “Geodesics in general relativity”, it says the following: “Thus, for example, the path of a planet orbiting a star is the projection of a geodesic of the curved 4-D ...
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I make two different substitutions (m(r)=m1(r), m2(r)) to the modified Schwarzschild metrics: $${\it m1} \left( r \right) =1/2\,r{c}^{2} \left( 1+\sqrt {{\frac {r{c}^ {2}-2\,MG}{r{c}^{2}}}} \right) {G}... 1 vote 0 answers 68 views ### What does the upper and lower indices in metric tensor means? When we have metric g that can be written as$$\begin{vmatrix}g_1 & g_2 & g_3\\ g_4 & g_5 & g_6 \\ g_7 & g_8 & g_9\end{vmatrix} What does the $g^{\alpha\beta}$ and $g_{\alpha\... • 111 1 vote 0 answers 86 views ### If astronomers didn't know about redshift could we still determine the universe is expanding? What other evidence is there for the expansion of the universe? Would the equations of physics look wrong without it? 1 vote 0 answers 36 views ### Modelling dark energy as a potential? Dark energy is the cosmological constant.However can we model dark energy as some form of field with a corresponding potential? • 111 1 vote 0 answers 55 views ### Does the (relative) distance of the photon sphere from the center of a black hole vary according its mass? I know that the photon circle or photon sphere is located at around 1.5 times the radius of the Schwarzschild event horizon, but wouldn't it be comparatively further out for smaller black holes? After ... • 5,177 1 vote 0 answers 68 views ### Non-homogeneous and anisotropic metric and laws of physics...? In this popular science article, they say that if our universe resulted to be non-uniform (that is highly anisotropic and inhomogeneous) then the fundamental laws of physics could change from place to ... • 1,017 1 vote 0 answers 27 views ### Spacetimes where symmetries vary from place to place? Are there spacetimes or metrics where symmetries (like Poincaré, Lorentz, diffeomorphism, translational... invariances) are only local and the symmetries of one local neighbourhood are not, a priori, ... • 1,017 1 vote 0 answers 130 views ### In between Classical Gravitational Deflection (2GM/rv^2) and General Relativity (4GM/rc^2) While researching online I've found two expressions for the angle of deflection by gravitational lensing:$\dfrac{2GM}{rv^2},$Classical(Newtonian) prediction$\dfrac{4GM}{rc^2},$Relativistic ... 1 vote 0 answers 35 views ### Does the Schwarzschild metric solution require two (A and B, or g11 and g44) or four components (g11, g22, g33 and g44)? Different places on the web imply that Schwarzschild's metric uses four components or separate equations, similar to how Einstein's full set requires 6 or 10, or they say only two are needed. Is it 2 ... • 5,177 1 vote 0 answers 50 views ### Why does Titan's (relatively) rapid migration away from Saturn cause the planet to wobble faster, and eventually (maybe) tip onto its side? Over the past year and a half, multiple articles in the popular press have come out talking about how unexpectedly rapidly Titan is moving away from Saturn, and how this is causing Saturn to wobble ... • 5,177 1 vote 0 answers 102 views ### What happens inside a massive collapsing shell? I have already asked this on the physics site. I am not sure where it fits better. Consider a massive spherical shell collapsing under its own gravity. Say the mass is that of one galaxy. The ... 1 vote 0 answers 60 views ### How much does the Higgs field contribute to the energy of empty space? The Higgs field was not spontaneously broken when the universe was young and hot (like I am not...:)). At the very high temperature that existed back then, the Higgs' energy of the Higgs field was non-... 1 vote 0 answers 70 views ### Questions on dark matter observations in galaxy clusters For cluster of galaxies we can observe velocity distributions and so we can deduce mass from multiwavelength observations. I suppose total mass of the cluster could be deduced by observing velocities ... 1 vote 0 answers 67 views ### is it possible to prove some version of the "cosmic censorship hypothesis" of Roger Penrose which proposes that this is impossible? Can singularities not hidden behind an event horizon, known as "naked singularities", arise from realistic initial conditions, or is it possible to prove some version of the "cosmic censorship ... • 359 0 votes 0 answers 22 views ### Is the hypothetical '(Don) Page curve' of evaporating black holes equal in time, so to speak, on both sides of the curve? I have heard that Hawking radiation is slow at first, but rapidly increases when the black hole becomes much smaller... (I forget the formula...) Does that mean that the second half of Page's curve, ... • 5,177 0 votes 0 answers 33 views ### Is the difference of redshift parameter z able to prove expansion is accelerating in flat FLRW universe model? Sorry for my bad english, this is not my mother tongue. Recently, I've encountered a problem in cosmology class that is about FLRW universe. Assuming that the universe we inhabit is composed of non-... 0 votes 0 answers 60 views ###$v_{GW}\$ near a binary star?

In empty space, at large distances from masses and at small amplitudes, GWs propagate at the speed of light. This does not apply near the source, because there is a large mass and the amplitude of the ...
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### Does the other side of the Big Bang factor into JWST observations?

Our (roughly) 13.6 billion light year view to the point of origin (big bang) is just along a radial axis. Assuming most matter ejected in a (roughly) spherical pattern, the diameter of the universe is ...
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### Does a gravitational lens around a body look 3-dimensional?

Suppose we approach a neutron star or a stellar black hole close enough, is the gravitational lens around it 3-dimensional (like a tunnel) or is it 2-dimensional? It's 3-dimensional as if you entered ...
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### How would you define "space" in GR (General Relativity) and in QFT (Quantum Field Theory)?

Is there a univocal definition of space that can be accepted in GR and in QFT? In GR space is actually "Spacetime", a 4-dimensional entity that includes time. In this case it's impossible to ...